1. Field of the Invention
The present invention relates to improved methods of preventing water and/or gas flow through a subterranean zone, and particularly to such methods wherein the subterranean zone has a high temperature and is at a depth requiring a long pumping time.
2. Description of the Prior Art
The production of water with oil and/or gas from wells constitutes a major problem and expense in the production of oil and gas. While oil and gas wells are usually completed in hydrocarbon producing formations, when there is a water producing zone adjacent to the hydrocarbon producing formation, the higher mobility of the water often allows it to flow into the hydrocarbon producing formation by way of natural fractures and high permeability streaks. In the production of such wells, the ratio of water to hydrocarbons recovered often becomes so high that the cost of producing the water, separating it from the hydrocarbons and disposing of it represents a significant economic loss.
In order to reduce the production of undesired water and/or gas from hydrocarbon producing formations, aqueous polymer solutions containing cross-linking agents have been utilized heretofore. Such aqueous polymer solutions have been pumped into the hydrocarbon producing formations so that they enter water and/or gas zones within and adjacent to the formations and cross-link therein. The cross-linking of the polymer solutions causes them to form stiff gels which stop or reduce the flow of the undesired water and/or gas.
While the use of aqueous polymer solutions for reducing the production of undesired formation water and/or gas has achieved varying degrees of success, the cross-linking agents used to cross-link the polymer have often contained chromium or phenol/formaldehyde. These cross-linking agents are undesirable in that they either have high toxicities or produce cross-linked gels having low gel stabilities at high temperatures and/or at pH values greater than about 8.
In addition, the aqueous polymer compositions utilized heretofore for reducing the production of undesired water and/or gas from subterranean zones having high temperatures and requiring long pumping times have often been unsuccessful. That is, the compositions have not produced gels which remain stable at the high temperatures involved and/or they have not had the required pumping time at the high temperatures to reach and be placed in the subterranean zone.
Thus, there is a continuing need for improved methods of preventing water flow through subterranean zones.